The gram-negative facultative Eikenella corrodens has been identified as a periodontal pathogen. This organism is also routinely cultured from the oral cavity of healthy subjects. The distinction between E. corrodens strains associated with health or disease has not been investigated. Interestingly, E. corrodens displays a wide spectrum of genetic, phenotypic and antigenic variations. Genetically different E. corrodens strains may simultaneously colonize the same subject. Previous studies assessing the humoral immune response against E. corrodens generally used one or two allegedly representative strains in the assay. Due to the heterogeneity of E. corrodens, the information obtained may be irrelevant. Further, no study has attempted to assess the variation of the immune response against different strains colonizing the same subject. Such studies may provide insight to the possible distinction between pathogenic and nonpathogenic E. corrodens. Consequently, the focus of the current research plan is to evaluate the serum antibody immune response against E. corrodens in 35 clinically well-defined subjects colonized by this organism. ELISA will be performed to determine the serum antibody titer against E. corrodens using strain(s) and serum from the same subject. Three commonly used E. corrodens representative Strains will be included in the assay to determine the possible discrepancy from using non-autologous strains in ELISA. The immunogenicity of E. corrodens antigens, particularly the major outer membrane proteins, lipopolysaccharides and slime extracts will be determined by immunoblotting. The current research plan will determine (1) the possible differences in the serum antibody levels against E. corrodens in periodontally healthy subjects, localized juvenile periodontitis patients and adult periodontitis patients, (2) the possible discrepancies between ELISA tiers determined using common reference strains and autologous strains, and (3) the possible difference in serum antibody levels of individuals against the various autologous but genetically distinct E. corrodens strains. In addition, the current research plan may identify genetically distinct autologous E. corrodens strains from the same individuals which may elicit either strong or weak host immune responses. These strains may be of interest in future investigation of pathogenicity of E. corrodens.